In the electronics industry, solder metal, soldering flux, solder paste and similar materials are used for mounting electronic parts on the surface of a substrate. Since solder paste is excellent in printing property and has good viscosity, it is suitable for automatic mounting and has recently been used in an increasing amount.
By screen-printing or with a dispenser, a solder paste is applied to a substrate, on which electronic parts are then mounted and to which the electronic parts are fixed through reflowing of the solder paste. The term “reflowing” used herein refers to a sequential process including the steps of preheating the substrate on which electronic parts have been placed and heating the substrate at a temperature higher than the melting temperature of the solder paste, thereby joining the parts to the substrate.
Recently, in order to keep pace with the trend for scaling down the size of electronic products, demand has arisen for electronic parts with fine pitches, and fine-pitch electronic parts, such as 0.3-mm-pitch quad flat package (QFP) type LSIs and chip size packages (CSPs), have currently been widely employed. Under these circumstances, solder paste is required to have high wettability for joining fine-pitch electronic parts so as to impart high thermal shock resistance to the joined products and to attain proper mounting of electronic parts on a substrate. In order to meet this demand in the industry, solder metal and solder paste must satisfy the above requirements.
However, when conventional solder alloy or solder paste is used for soldering fine-pitch or large-scaled electronic parts recently demanded, wettability and thermal shock resistance are unsatisfactorily attained, thus deteriorating reliability of joining portions. In addition, because of recent environmental concerns in relation to lead, use of Pb-free solder is being encouraged and accordingly development of Pb-free solder is under way.
Sn—Ag based, Sn—Cu based, Sn—Bi based and Sn—Zn based solder members have attracted attention as promising ones, because these Pb-free solder members are advantageous in terms of melting temperature, wettability and reliability in joining. However, the solder members have drawbacks. Sn—Bi based solder members are fragile though they are advantageous in terms of melting temperature. Therefore, Sn—Bi based solder has not been used in practice. Since Sn—Cu based and Sn—Ag based solder members have excessively high melting temperatures, they when used raise a problem that electronic parts to be soldered are affected during soldering. Sn—Ag based solder members cost more than conventionally used Sn—Pb solder because they contain expensive Ag. In addition, excessive use of Ag results in depletion of resources and, when released to the environment, might affect the ecological system in view of life cycle assessment (LCA).
Sn—Zn based solder has attracted attention as promising Pb-free solder, because the solder has a melting temperature most similar to that of conventional Sn—Pb eutectic alloy. To Sn—Zn based solder, Bi is added in order to further lower the melting temperature thereof and improve wettability. However, a recent study has revealed that addition of Bi to Sn—Zn based solder lowers thermal shock resistance of joined products obtained by use of the solder. Wettability of Sn-9Zn eutectic alloy can be improved by, for example, employment of flux. However, the problem of the chip-standing phenomenon arises upon mounting of electronic parts.
Conventional Sn—Pb based solder members have high stability, by virtue of a considerably small difference in oxidation potential between Sn and Pb. In contrast, in the aforementioned Pb-free solder members, Sn exhibits a considerable difference in oxidation potential from Ag, Cu or Zn, thereby promoting oxidation of solder metal, resulting in a decrease in solderability. In particular, soldering by use of Sn—Zn based solder in the air has been difficult, since Zn has an oxidation potential lower than that of Pb, resulting in severe oxidation. In order to perform soldering in the air, a large amount of strong active agent must be incorporated into flux. In this case, solder metal readily reacts with the flux, and this reaction deteriorate the stability of the solder paste employed in the soldering.
In view of the foregoing, one object of the present invention is to provide solder metal and solder paste exhibiting improved wettability and attaining high thermal shock resistance of joined portions when soldering fine-pitch or large-scaled electronic parts onto a substrate.
Another object of the invention is to provide Sn—Zn—Bi solder metal and solder paste capable of suppressing a decrease in thermal shock resistance of portions to be joined.
Still another object of the invention is to provide soldering flux that can suppress reaction with solder metal to obtain highly stable solder paste.